1. Field of the Invention
The present invention relates to a device for combining electron waves traveling along a plurality of waveguides into a single electron wave or inversely for branching an electron wave traveling along a waveguide into a plurality of electron waves using field-induced decoupling. Quantum interference devices utilize such combining and/or branching devices in which interference currents can be effectively modulated by external perturbations such as a magnetic flux, an electrostatic potential, or virtual transitions due to optical fields.
2. Related Background Art
There has been developed such quantum interference devices as shown in FIG. 1. Such a device includes structures for combining electron waves and/or branching the electron wave. In this device, the left half is an electron wave branching part and a right half is a combining part. The electron wave is input from an electrode 201 and propagates as a single electron wave in a part where the distance between two electron wave channels 202 and 203, having the same structure, is short enough to cause the coupling of electron waves. The electron wave then reaches a decoupling or branching part. In this branching part, the distance between the channels 202 and 203 is made larger than that in the upstream coupling part, so that a single electron wave is branched into two therein. These coupling and decoupling are in fact realized by changing the thickness of a barrier layer 204 comprising an AlAs layer.
Two branched electron waves then arrive at a combining part on the right side, are coupled into a single electron wave again and proceed as this single electron wave to finally each an electrode 205. A magnetic field H is applied to the decoupling part, having a direction as shown in FIG. 1. Therefore, when the magnetic field H is applied having such a magnitude as to cause a phase difference of an integer multiple of .pi. between the electron waves that respectively proceed along the upper and lower channels 202 and 203, the two electron waves which interfere with each other in the right-side combining part will all be reflected and therefore could not reach the electrode 205. When the applied magnetic field H has such a magnitude as to cause the phase difference of an integer multiple of 2 .pi., the interfered electron waves can pass through the right-side combining part and an output current having the same magnitude as the input current can be obtained from the electrode 205. Thus, in this device the current is controlled by modulating the applied magnetic field H. This is due to the magnetostatic Aharonov-Bohm (AB) effect. This current can also be controlled by modulating an electric field applied normal to the plane of channels 202 and 203, due to the electrostatic AB effect. In light of the structures of the channels 202 and 203, they are fabricated in such a fashion that the above phase difference can be caused by the AB effect.
As regards the length between the branching and combining parts in FIG. 1 (i.e., the length during which the decoupling is maintained), this should be less than 1 .mu.m at temperatures of liquid helium under the present-day technology, since the coherence of electron waves should be maintained over that length.
The above device is the AB-effect type electron wave interference device, but there are some other types such as a resonant tunneling type, and an electron wave directional coupler type. FIG. 2 shows an electron wave interference device of the directional coupler type in which two electron wave channels 211 and 212 are fabricated on a semiconductor surface by etching, etc. In the channels 211 and 212, they are connected to each other at a central part or coupling part 213 and are separated and independent from each other at port parts 215, 216, 217 and 218. An electrode 219 is formed near the coupling part 213 to control the degree of coupling between two channels 211 and 212. In this connection, it should be noted that the coupling of electron waves should be distinguished from the coupling of channels or waveguides. For instance, the electron wave may be decoupled in the coupled channels.
In the prior art device of FIG. 2, the modulation in the degree of coupling is performed by controlling the electrostatic potential or voltage applied to the electrode 219 to vary the height of potential barrier between the two channels 211 and 212. This modulation in the degree of coupling in turn varies the coupling length between the channels 211 and 212 and the propagating electron wave will be switched between the port parts 215-218.
However, those prior art devices have the following drawbacks.
First, in the structure of FIG. 1, it is hard to fabricate slope portions of the barrier layer 204 in the branching and combining parts so that phases of the electron waves may not be confused. It is difficult to fabricate an extremely flat electron wave waveguide (fluctuations of thickness are below 1 atomic layer). Moreover, it is difficult to fabricate the slope portions because very high-precision device fabrication technology is needed. In particular, in order to form the structure of FIG. 1, layers to the AlAs barrier layer 204 between the two channels 202 and 203 are to be grown by the molecular beam epitaxy (MBE) process or metal organic-chemical vapor deposition (MO-CVD) process. It is then necessary to take out the wafer in the air and conduct the etching to form thick and thin portions in the barrier layer 204. But, at this time oxidization and impurity absorption at the surface of wafer will be inevitable. Therefore, the channel 203, such as GaAs quantum well, which is to be grown on this surface of wafer, would have a dilapidated hetero-boundary. This surface would then contain many impurities and defects. As a result, electrons which travel through that channel would be scattered and its phase would be disturbed, resulting in the degradation of the on/off or modulation ratio and detection performance in the interference current.
Further, in the structure of such device, the response of the device then depends on the RC time constant which effects the modulation of magnetic field, etc., so that very high speed response rapidity cannot be obtained.
Next, in the device of FIG. 2, provided that the wavelength of electron waves is several hundred .ANG. and the coherence length of electrons is several .mu.m (this assumption is reasonable), it is needed to set the widths of the channels 211 and 212, the length of the coupling part 213 and the width of the electrode 219 to about 100 .ANG., 1 .mu.m and several tens .ANG., respectively. But, these values are unfeasible or impractical under the present-day device fabrication technologies.
Further, the electron wave channels 211 and 212 are curved, as a result of which the electron waves would be scattered and its phase would be disturbed. Thus, the performance of such prior art interference device would be degraded.